Increased running speeds of paper and board machines cause new problems to be solved, which problems are mostly related to the runnability of the machine. At present, speeds of up to about 1600 to 1700 meters per minute are employed in paper machines. At these speeds, so-called closed press sections, which comprise a compact combination of press rolls fitted around a smooth-faced center roll, as a rule, still operate satisfactorily. As examples of such press sections should be mentioned the current assignee's Sym-Press II.TM. and Sym-Press O.TM. press sections.
In recent years, as running speeds of printing-paper machines, running speeds as high as about 40 meters per second=2400 meters per minute have been contemplated. Achievement of speeds as high as this, in particular in wide machines, provides ever more difficult problems to be solved, some of the most important of which are runnability of the machine and adequate water drain capacity at a high web speed. In a corresponding way, in board machines (basis weight of web&gt;100 grams per sq. meter), attempts are made to raise the present-day web speeds (8 to 15 meters per second) to a level of 15 to 25 meters per second.
When running speeds of paper machines are increased, the problems of runnability of a paper machine are also encountered with increased emphasis, because a web with high water content and low strength especially does not endure the dynamic forces arising from high web speeds and changes in the running direction of the web, but web breaks and other interference in the operation arise, which will be described in more detail later and which produce standstills or faults of quality in the paper. In a modern printing-paper machine, the cost of break standstill time is currently of an order of 50,000 FIM per hour.
In prior-art press sections, the web is, as a rule, passed from the forming wire into the first press nip on a pick-up felt, which also operates as a press fabric that receives water in the first press nip, which nip is either a roll nip or an extended nip. In the first press nip, it is necessary to use a relatively high compression pressure and to deal with large quantities of water, and it is one drawback arising from this that the outer face of the press felt tends to be contaminated and its porous fabric structure to be partly blocked. Attempts are made to prevent this by means of efficient felt conditioning devices, which are, however, components that are quite expensive, spacious, and consume an abundance of energy. A further drawback of the press felts used in prior-art press sections is their effect of rewetting the web.
In a way known from the prior art, in the vicinity of the wet wire, before the pick-up point, devices for cutting of edge strips off the web, i.e. so-called edge spray cutters, are fitted, by whose means, by means of water jets applied to the web placed on the wet wire, narrow edge strips are cut off from both edges of the web so that a web of uniform width with whole and good edges is obtained to be passed to the press section. As is known from the prior art, the cutters of edge strips can be provided with position regulation devices, by whose means the web cutting width can be adjusted and regulated within certain limits.
By means of the present invention, attempts are made to eliminate problems related to the cutting of the edges of the paper web which have been encountered in the current assignee's test machine when running at a high speed and in particular with webs of relatively low basis weight. When the running speed of the test machine was higher than about 1500 meters per minute, the edge strips that were cut off from the paper web in the wire part before the pick-up roll tended to follow the pick-up felt after the pick-up roll, instead of following the wet wire. This took place in particular when the permeability of the pick-up felt was low. The permeability of the pick-up felt can be low also when new, because of its structure, and in particular when the felt is compacted and contaminated as a result of prolonged service life, as was described above. If the edge strips of the paper web enter fully or partially into the press nips, they cause interference with the runnability and produce fiber strings that damage the press felts and the extended-nip belts.
The above problems related to the cutting of the edges of a paper web and the other difficulties are emphasized further when the web speed approaches 2000 meters per minute and exceeds the speed. It is one of the problems of runnability that, owing to these problems, in the press section it is not possible to employ steam boxes in order to enhance the dewatering. It has not been possible to a major extent to reduce these problems by means of regulation of the cutting width of the water jet devices that cut the edges of the paper web, i.e. of the edge spray cutters, or by means of regulation of the suction width in a pick-up roll.
It is probably one of the reasons for the problems related to the cutting of the edges of the paper web that replacement air flows into the opening nip between the forming wire and the pick-up felt more readily through the forming wire than through the pick-up felt. Since the width of the strip torn off from the edge is approximately equal to the width of the end seals of the suction zone in the pick-up roll, these seals may prevent the flow of replacement air through the holes in the mantle of the pick-up suction roll at the location of the seals. This is also indicated by the fact that it can also be frequently seen in production machines that the edge strips are folded double across a width of about 30 mm after the pick-up point before the edge strip knock-off jets.